The present invention relates to the field of air/fuel ratio control devices for internal combustion engines such as those internal combustion engines used for automotive vehicles, and more particularly relates to the field of such air/fuel ratio control devices for internal combustion engines which are equipped with double barreled carburetors in their fuel intake systems and three way catalytic converters in their exhaust systems.
Three way catalytic converters for internal combustion engines are per se well known in various different forms. Such a three way catalytic converter is capable of converting HC, CO, and other products of incomplete combustion in the hot exhaust gases of the internal combustion engine into harmless end products by an oxidizing reaction, and also of simultaneously converting nitrogen oxides (so called NOx) in the exhaust gases into harmless end products by a reducing reaction, provided that the air/fuel ratio of the exhaust gases passing into said three way catalytic converter is maintained within a rather narrow range about the stoichiometric condition. If, however, the air/fuel ratio of the exhaust gases passing into said three way catalytic converter wanders towards the lean side of stoichiometric, then although the above detailed oxidizing reaction for converting HC, CO, and other products of incomplete combustion in the hot exhaust gases of the internal combustion engine into harmless end products continues, the reducing reaction for converting nitrogen oxides in the exhaust gases into harmless end products will substantially cease; and, if the air/fuel ratio of the exhaust gases passing into said catalytic converter wanders towards the rich side of stoichiometric, then although the reducing reaction for converting nitrogen oxides in the exhaust gases of the internal combustion engine into harmless end products continues, the oxidizing reaction for converting HC, CO, and other products of incomplete combustion in the hot exhaust gases into harmless end products will substantially cease.
It is possible to control the air/fuel ratio of the exhaust gases passing into the three way catalytic converter within a narrow range about the stoichiometric condition by controlling the air/fuel ratio of the air-fuel mixture being supplied to the internal combustion engine through its intake system within a narrow range about the stoichiometric condition, and therefore conventionally many different sorts of fuel/air ratio control systems have heretofore been proposed which have as their goal maintaining the air/fuel ratio of the air-fuel mixture being supplied to the internal combustion engine close to the stoichiometric condition.
A typical such prior art system has an oxygen sensor fitted to the exhaust manifold of the internal combustion engine, upstream of the three way catalytic converter, so as to sense the presence of oxygen in the exhaust gases therein. The signal from this oxygen sensor is then sent to a device which provides extra air into the intake system of the engine at some point therein. In this case, the basic air/fuel ratio of the air-fuel mixture provided by the carburetor of the internal combustion engine is set to be rather on the rich side of stoichiometric, and thus by addition of a proper amount of extra air to the intake system and air/fuel ratio of the air-fuel mixture provided to the internal combustion engine may be controlled to be substantially the stoichiometric air/fuel ratio. Conventionally, the extra air can either be added directly into the intake manifold of the engine, downstream of the carburetor; but it is better from the point of view of mixing of air and of fuel for the extra air to be provided into a passage of the carburetor as an additional amount of bleed air to be mixed with the fuel being provided by the carburetor, in a per se well known fashion. In either case, by feedback control performed by the extra air control device based upon the signal from the oxygen sensor, the air/fuel ratio of the air-fuel mixture provided into the cylinders of the internal combustion engine can be satisfactorily controlled to be substantially the stoichiometric air/fuel ratio, and thereby the air/fuel ratio of the exhaust gases passing into the three way catalytic converter can be satisfactorily maintained within a narrow range about the stoichiometric condition.
This kind of prior art feedback system is effective in the case of a single barreled carburetor, but in the case of a double barreled carburetor, the use of which is becoming more and more frequent nowadays, certain difficulties tend to arise which will now be outlined.
Such a double barreled type of carburetor is provided with a main or primary air intake passage and fuel supply system and a secondary air intake passage with its own fuel supply system. A primary throttle valve is mounted in the primary air intake passage so as to control its opening amount, and a secondary throttle valve is mounted in the secondary air intake passage so as to control its opening amount. Conventionally the primary throttle valve is opened and closed according to the depression of an accelerator pedal or the like of a vehicle to which the internal combustion engine incorporating the carburetor is fitted, and the secondary throttle valve remains closed until the primary throttle valve is opened to a predetermined throttle opening amount, and then, provided that the intake air flow is greater than a certain predetermined air flow amount, opens progressively as the primary throttle valve opens beyond said predetermined opening amount.
The question therefore arises as to at what place in such a double barreled carburetor with two fuel supply systems the extra bleed air, described above, regulated by the extra air control device based upon the signal from the oxygen sensor, should be injected. A system such as for the primary fuel system of the carburetor to have one bleed air supply system incorporating its own primary extra or bleed air control device and for the secondary fuel system to have its own independent secondary bleed air supply system also incorporating its own secondary extra or bleed air control device (the two systems may of course share the same oxygen sensor in the exhaust system of the engine) would solve this question satisfactorily. In this case, air/fuel ratio control would be performed for both the primary fuel system and also the secondary fuel system independently, and accordingly both the air/fuel ratio of the air-fuel mixture produced by the primary fuel system would be kept within a reasonably small range around the stoichiometric value and also the air/fuel ratio of the air-fuel mixture produced by the secondary fuel system would be kept within a reasonably small range around the stoichiometric value. Further, during transient operating conditions such as quick opening or closing of the primary and secondary throttle valves the deviations from the approximately stoichiometric air/fuel ratio of the air-fuel mixture provided by the primary and secondary fuel systems would not be very great. However, the disadvantages of such a system are that two control devices are necessary, and this causes the amount of mechanism to be large, and the cost and the bulk of the system also becomes excessive.
Further, as a general principle, since in fact the secondary throttle valve is not actually opened very often in normal vehicle operation, it is really rather wasteful to provide a special secondary extra or bleed air control device just for the secondary air bleed control system.
An alternative system that has been practiced in the prior art is, therefore, for the primary fuel system of the carburetor to have a bleed air supply system incorporating a primary bleed air control device, and for air bleeding control to be carried out only on the primary fuel supply system of the carburetor, and not on the secondary fuel supply system at all. This system of course avoids the disadvantages outlined above of high cost and duplication of mechanism, and of course when the primary throttle valve is opened but the secondary throttle valve is not opened the regulation of the amount of bleed air, in the above mentioned feedback manner, is performed properly. However, when both the primary throttle valve is opened and also the secondary throttle valve is opened, because the air/fuel ratio of the air-fuel mixture supplied by the carburetor as a whole must be kept substantially at the stoichiometric value by the above described feedback operation, with air bleeding only being performed into the primary fuel supply system, the air/fuel ratio of the air-fuel mixture supplied by the primary fuel supply system will be substantially higher, i.e. leaner, than stoichiometric, while the air/fuel ratio of the air-fuel mixture supplied by the secondary fuel supply system needs to be maintained less, i.e. richer, than stoichiometric.
Now, provided that the air-fuel mixture which is being produced by the primary fuel supply system is well mixed with the air-fuel mixture which is being produced by the secondary fuel supply system before being distributed between the cylinders of the engine, under normal operating conditions of the engine the system will operate correctly in the feedback manner outlined above. However, when for example the vehicle incorporating the engine is quickly decelerated and the accelerator pedal thereof is released quickly from the above described high load condition in which both the primary throttle valve and also the secondary throttle valve are open, i.e. the secondary throttle valve is quickly closed to its completely closed position and the primary throttle valve is still opening at an opening amount substantially less than the aforementioned predetermined opening amount, then the operation of the secondary fuel supply system, which was supplying air-fuel mixture of air/fuel ratio substantially richer than stoichiometric, stops immediately, but the operation of the primary fuel supply system, which was supplying air-fuel mixture of air/fuel ratio substantially leaner than stoichiometric, continues; and, during the inevitable time delay interval before the above described feedback system brings the opening of the bleed air control valve to the equilibrium opening amount which provides an air/fuel ratio for the air-fuel mixture being supplied by the primary fuel system of approximately stoichiometric (this time delay is inevitable because of the time taken for the physical elements of the bleed air control valve to move to their new positions, as well as other factors), the air/fuel ratio of the air-fuel mixture being supplied by the primary fuel supply system is the same as it was while the secondary fuel supply system was operating, i.e. is substantially larger or leaner than stoichiometric; and during this time delay interval therefore a substantially leaner air-fuel mixture than stoichiometric is supplied to the internal combustion engine. In this over lean engine operation condition the very undesirable consequences are liable to occur of deterioration of engine drivability and also of increase in the emission of nitrogen oxides or NOx in the exhaust gases of the engine, due to the poor operation of the three way catalytic converter in its mode of removing nitrogen oxides from the exhaust gases by a reducing reaction in the state of the exhaust gases of containing an excess of oxygen, i.e. of being over lean.